Translucent polycrystalline alumina (PCA) ceramic has made possible present-day high-pressure sodium (HPS) and ceramic metal halide lamps. In HPS lamps, the typical wall temperature is 1250° C. in the center of the arc discharge tube, and 800° C. at the cold end. Typically, the operating life of an HPS lamp exceeds five years. In contrast, although the wall temperature of the arc discharge vessel in ceramic metal halide lamps (1000-1150° C.) tends to be somewhat lower than in HPS lamps, the operating life of the lamp is typically less than one year. This is because the PCA is corroded by the rare earth halide fills of metal-halide lamps. Hence, the durability and life of such lamps are significantly less than HPS PCA lamps.
Construction of the PCA vessel for a ceramic metal-halide lamp ranges from cylindrical to bulgy. Examples of these types of arc discharge vessels are given in European Patent Application No. 0587238A1 and U.S. Pat. No. 5,936,351, respectively. The bulgy shape yields a more uniform temperature distribution, resulting in reduced corrosion of the PCA by the lamp fills. Even so, the operating life of metal-halide lamps made with bulgy-shaped PCA discharge vessels has been reported to be two years, which is still significantly less than the 5-year life of PCA arc tubes in HPS lamps.
Aluminum nitride (AlN) has been shown to be more resistant to the corrosive effects of rare earth metal halide fills. The use of AlN arc discharge vessels for ceramic metal halide lamps is described in European Patent Application No. 0371315A1 and PCT Application No. WO 03/060952. However, it is difficult to make aluminum nitride ceramics into fully-dense, high-transmittance parts to meet the requirements for lamp applications. AlN has a wurtzite structure with hexagonal symmetry. Because of the anisotropy of AlN grains, and the grain-boundary phases (Ca—Al—O phases, nitrogen-containing calcium aluminate phases and Al—O phases such as Al2O3—N and Al2CO) derived from the sintering aids (calcia, CaO) and surface oxide on AlN powders, the fully-dense sintered AlN ceramic is only translucent and not transparent. Both the total and in-line transmittance values of as-sintered AlN arc discharge vessels are generally lower (˜75% and 0.2%, respectively) than those of PCA arc tubes (˜98-99% and 8%, respectively). The in-line transmittance is not of concern for diffuse lighting applications such as outdoor or indoor, wide-area illumination. However, the low total transmittance is a major concern, since it strongly affects lumen output. Arc discharge vessels typically require >92% total transmittance in the visible wavelength region from about 400 nm to about 700 nm in order to be useable in commercial lighting applications.